Since there is one 7 chip and one 15 chip
If you expect to pick a 7 chip, the probability is 1/15.
If you expect to pick a 15 chip, the probability is also 1/15.
The word "or" is simply used in addition rule of probability. Therefore,
P(7 or 15) = 1/15 + 1/15 = 2/15
